The present invention relates to digital-to-analog conversion apparatuses or DA conversion apparatuses utilizing oversampling technology and noise shaping technology based on delta-sigma modulation.
FIG. 7 shows an example of the conventional DA conversion apparatus utilizing the combination of oversampling technology and noise shaping technology. In FIG. 7, the DA conversion apparatus is comprised of a digital filter 10 for oversampling a multi-bit digital input DI to produce an oversampled digital signal A, a delta-sigma modulator 12 for effecting delta-sigma modulation, i.e., differentiation/integration processing of the oversampled multi-bit digital signal A to reduce a number of bits to feed a requantized digital signal B of lesser number of bits, a waveform shaper 14 for waveform-shaping pulses of the requantized digital signal B in synchronization with a clock signal to produce a shaped pulse signal C, a clock generator 16 for generating a system clock signal .phi.s having a given frequency fs, and a low-pass filter or LPF18 for filtering the pulse signal C to convert the same into an analog output AO corresponding to the digital input DI.
A circuit portion enclosed by the dot-and-chain line IC is comprised of an integrated circuit of the monolithic or hybrid construction disposed in a single package, while a quartz resonator 16A is externally coupled to the clock generator 16 as an oscillating source. Otherwise, the digital filter 10 and its associated parts, if any, enclosed by the broken line may be incorporated into the same integrated circuit.
The digital input DI is given in the form of a digital waveform containing, for example, a sample sequence of digital data each composed of 16 bits, i.e., one word, and being fed to the digital filter 10 at, for example, 44.1 KHz of relatively low data transfer rate. The system clock signal .phi.s has, for example, 16.9 MHz of high frequency fs. The oversampled digital signal A is fed from the digital filter 10 to the delta-sigma modulator 12 at, for example, 8.45 MHz (fs/2) of high data transfer rate.
The faster the oversampling rate, the better the filtering performance of the digital filter 10. However, the heavy increase of the oversampling rate may adversely affect overall design of the DA converter. In view of this, the delta-sigma modulator 12 is provided to efficiently lower an oversampling frequency or rate in the oversampling operation at the digital filter 10. The delta-sigma modulator 12 effects delta-sigma modulation of the oversampled digital signal to requantize the same such that the requantization error is significantly distributed in a higher frequency range to thereby noise-shape the digital signal. For this reason, the delta-sigma modulator 12 is called a noise shaper. The delta-sigma modulator 12 may be of single or double loop type constructed such as to produce the requantized and noise-shaped signal B in the form of a pulse-density-modulated signal or a bit stream signal. Otherwise, the delta-sigma modulator 12 may be of triple or more loop type constructed such as to produce a pulse-width-modulated signal B. The delta-sigma modulator 12 operates to convert a multi-bit digital signal to a reduced multi-bit digital signal having a lesser number of bits. Such conversion of modulation may cause a quantization error or distortion which may develop significantly in a relatively high frequency range.
Namely, FIG. 9 indicates a power spectrum of an ideal output of the delta-sigma modulator 12. The power spectrum contains a sharp peak Ps at the operating clock frequency fs of the delta-sigma modulator 12, and a maximum noise power at the vicinity of the data transfer frequency fs/2. Though not shown in FIG. 9, this spectrum is repeatedly observed in every range of 1fs, 2fs, 3fs and so on. Further, white noise may be actually superposed on the power spectrum although not shown in the FIG. 9 graph.
The noise-shaped signal B may still contain various noise factors due to fluctuation or distortion during digital requantization processing. Therefore, if the digital signal B were directly converted into an analog output by the LPF18, there would be caused a considerable error due to the noise factors. In view of this, the waveform shaper 14 is interposed to waveform-shape the requantized signal B based on the system clock signal .phi.s to thereby feed the wave-form shaped signal C to the LPF18 in order to reduce error due to the various noise factors.
The waveform shaper 14 carries out the waveform-shaping operation as indicated by a timing chart shown in FIG. 8. Namely, an AND operation is effected between the requantized signal B representative of a sequence of binary data, e.g., 1, 0, 1, 1,--, and a shaping clock signal formed in synchronization with the system clock signal .phi.s so as to produce the waveform-shaped pulse signal C comprised of a train of pulses representative of the same sequence of binary data 1, 0, 1, 1,--.
In the above noted conventional DA conversion apparatus, when the digital input shifts to a considerably small level, the requantized signal approaches a repeated pattern of 1, 1, 0 and 0 according to the delta-sigma modulation such that an intensive noise spectrum component Po develops at the frequency fs/2 as shown in FIG. 9. Further, another noise spectrum component also develops at the vicinity of the frequency fs/2 due to the internal processing in the delta-sigma modulator.
Therefore, these noise spectrum components at and around the frequency fs/2 are leaked back into the clock generator 16 as a noise. For this, the clock generator 16 produces an output spectrum, as shown in FIG. 10, which contains subsidiary frequency components indicated by dashed lines at and around the frequency fs/2 besides the true oscillating frequency component fs.
Similar noise leaking phenomenon may occur when the oversampled digital signal A is fed to the delta-sigma generator 12 at the transfer rate of fa=fs/2, such that a noise component at and around the frequency fs/2 is leaked into the system clock signal .phi.s. In this case, the digital signal A transmitted from the digital filter 10 at the transfer rate of fa=fs/2 partially travels through a space into a junction terminal of the quartz resonator 16A so that the system clock signal output is mixed with a noise frequency component at and around fs/2 in manner similar to the FIG. 10 case.
In the afore-mentioned logical AND operation for the waveform-shaping, multiplication processing is effected between the requantized signal B and the system clock signal .phi.s. Generally, multiplication operation of two waveforms having frequencies .omega..sub.1 and .omega..sub.2 is represented in the typical form of sin .omega..sub.1 t.times.sin .omega..sub.2 t=-1/2{cos(.omega..sub.1 +.omega..sub.2)t-cos(.omega..sub.1 -.omega..sub.2)t}. The resulting waveform has two new frequencies .omega..sub.1 +.omega..sub.2 and .omega..sub.1 -.omega..sub.2. This relation can be applied to multiplication of two noise frequency components to thereby cause a folded noise at a differential frequency range (.omega..sub.1 -.omega..sub.2) and a summed frequency range (.omega..sub.1 +.omega..sub.2). In this particular case, a folded noise is generated by the multiplication processing between the noise spectrum component of the requantized signal B at and around the frequency fs/2 shown in FIG. 9 and the noise frequency component of the system clock signal .phi.s at and around the frequency fs/2 shown in FIG. 10. Particularly, some folded noise corresponding to a differential frequency therebetween develops in the audio frequency range R (FIG. 9) to thereby hinder S/N ratio of an audio analog output.